a) Field of the Invention
The invention is directed to a directly modulatable laser comprising a laser medium inside a laser cavity.
The arrangement is used for direct modulation of the emission of a laser, particularly a solid state laser or fiber laser or a reamplified solid state laser or a reamplified fiber laser.
b) Description of the Related Art
Fiber lasers are known, for example, from R. G. Smith, Appl. Opt. 11, 2489 (772), H. Po, et al. xe2x80x98High Power Neodymium-doped Single Transverse Mode Fibre Laserxe2x80x99, Electronics Letters, Vol. 29, No. 17, p 1500 (793) and P. Urquhart xe2x80x98Review of rare earth doped fibre lasers and amplifiersxe2x80x99, IEE Proceedings, Vol. 135, Pr. J, No. 6, December (788). In order to modulate the light emission of a fiber laser, it is possible to modulate the light output of the pump light source. However, with this method only modulation frequencies below the relaxation frequency of the fiber laser can be achieved, namely, in general, only several tens to hundreds of kilohertz. Modulation can be improved by continuous pumping of the fiber laser and reamplification of a rapidly modulatable external signal source at the emission wavelength of the fiber laser (seed) in the laser fiber.
The rapidly modulatable signal source can be effected from a laser diode modulated by the injection flow, a Q-switched laser or continuous-wave laser with intensity modulators in the light path between the signal light source and the fiber laser. However, this method leads to low modulation depths because the amplified spontaneous emission (ASE) of the fiber laser leads to a reduction in contrast when the signal light source is switched off. Further, a temporary exaggerated or elevated power of the output signal occurs when the signal light source is switched on after a long dark period because of the elevated inversion in the active fiber which builds up during the dark period.
This can be prevented by a two-wavelength, two-polarization or two-direction method described in the German Patent Application DE 198 29 684 A1. The fiber amplifier is kept constantly in saturation by switching over or modulating emission light of two wavelengths, two polarizations or two radiating directions through the fiber laser in such a way that the sum of the two light outputs remains constant. In this way, unwanted ASE is completely suppressed and high contrast is achieved. An inversion elevation after dark periods is prevented. The switching over or modulation of the signal light is carried out by modulating the injection flow of two laser diodes serving as seeds.
The disadvantage in this method is that the emission wavelengths of the signal source and fiber laser must match. This is not always possible when using a diode laser as signal source, particularly in the visible spectral range. Further, coupling two laser diodes into a monomode fiber requires complicated opto-mechanical precision elements. It is not possible to provide compact, adjustment-free systems comprising pump light sources, changeover switches or modulators and fiber lasers.
It is the primary object of the invention to provide a compact component group as a combination of a solid state laser or fiber laser or reamplified solid state laser or reamplified fiber laser and an internal modulation arrangement which is capable of delivering intensity-modulated laser light up to extremely high modulation frequencies and high light outputs. Further, the invention should solve the problem of insufficient output strength of individual components of modulatable solid state lasers, fiber lasers and amplifiers. Further, a modulation of the light of at least one wavelength should be carried out in such a way that its intensity curve follows the applied electric modulation function more accurately than was possible heretofore.
The invention relates to a directly modulatable laser comprising an active medium inside a laser cavity formed by a resonator mirror and an out-coupling mirror, and a pump light source exciting the active medium.
The invention is characterized, in a first instance, in that the active medium generates radiation of two wavelengths xcex1 and xcex2 and the resonator mirror is constructed as a controllable reflector by which the reflectivity is controllable for each of the two wavelengths xcex1 and xcex2 and the controllable reflector is connected with a control unit, wherein the reflection factor is controlled in such a way that the inversion density of the electrons which is generated in the active medium is constant and the output of one of the wavelengths is controllable between a minimum value and a maximum value according to an applied control signal E, wherein the control of the two wavelengths xcex1 and xcex2 is carried out in push-pull. This arrangement is also known as a directly modulatable laser according to the two-wavelength method.
In a first construction, the two emission wavelengths are generated in a broad-band amplifying medium, wherein these emission wavelengths are selected by the configuration, according to the invention, of the laser resonator or cavity.
In a second construction, the two emission wavelengths are generated in an active medium amplifying on two different lines.
The invention is described in this case only for two wavelengths. Of course, the directly modulatable laser can also be operated with three or more wavelengths, wherein the basic ideas of the invention must be utilized. Accordingly, in this case, the fiber laser is operated with two wavelengths. Because of the wavelength-selective characteristics of the reflectors, the resonance condition in the laser is given only for the wavelengths that are predetermined by the construction of the reflectors. The laser accordingly emits only on these two wavelengths. The amplification factor is adjusted by means of controlling the reflection factor, this control being independent for both wavelengths. In general, the control can be carried out in such a way that the sum of the emission outputs of the two wavelengths is constant.
The controllable reflector is used in this case as a wavelength-selecting resonator mirror of the laser or of a reamplified laser. The controllable reflector contains at least one modulator for the phase position and/or the polarization and/or the optical power of two light components. The effect or action of an intensity modulation for the two emission wavelengths or for only one of these wavelengths can be utilized at the output of the laser or reamplified laser.
The two emission wavelengths are not taken from external signal sources in this case, but from the active-ion doped laser fibers themselves, whose wavelength spectrum encompasses both emission wavelengths, or an actively doped fiber is used which can emit two discrete wavelengths. The amounts of the two emission wavelengths are determined in the two-wavelength method by the construction of the wavelength-selecting resonator mirror and the emission spectrum of the active fiber.
In a second instance, the invention is characterized in that the active medium generates radiation of a wavelength with two polarization directions P1 and P2 and the resonator mirror is constructed as a controllable reflector by which the reflectivity is controllable for each of the two polarizations P1 and P2 and the controllable reflector is connected with a control unit, wherein the control of the reflection factor is carried out in such a way that the inversion density of the electrons which is generated in the active medium is constant and the output of one of the polarizations is controllable between a minimum value and a maximum value according to an applied control signal, wherein the control of the two polarization directions P1 and P2 is carried out in push-pull.
This arrangement is also known as a directly modulatable laser according to the two-polarization method.
The controllable reflector is used in this case as a polarization direction-selecting resonator mirror of the laser or of the reamplified laser. The controllable reflector contains at least one modulator for the phase position and/or the polarization and/or the optical power of two light components.
The effect of a polarization modulation can be utilized for the two emission wavelengths or for only one of these wavelengths at the output of the laser or of the reamplified laser.
In the two-polarization method, two polarizations of the emitted light can be taken from an unpolarized active fiber which can be designed as polarization-preserving or non-polarization-preserving fibers. In the latter case, it may be required to monitor the double refraction or birefringence of the fiber.
In a third case, the invention is characterized in that the active medium generates radiation of one wavelength and the resonator mirror and out-coupling mirror are constructed in each instance as controllable reflectors by which the direction R1 and R2 of the light radiation is controllable and each of the controllable reflectors is connected with a control unit, wherein the control of the reflection factor is carried out in such a way that the inversion density of the electrons which is generated in the active medium is constant and the output of one of the directions is controllable between a minimum value and a maximum value according to an applied control signal, wherein the control of the two directions R1 and R2 is carried out in push-pull.
This arrangement is also called a directly modulatable laser according to the two-direction method.
A solid body or a light-conducting fiber is particularly suitable as active medium. However, dyes or gases can also be used as active media.
An advantageous construction of the directly modulatable lasers consists in that the selection of the two wavelengths xcex1 and xcex2 or of the two polarizations P1 and P2 is carried out after the output of the laser, i.e., after the light components have exited the laser cavity through the out-coupling mirror.
In a further development of the directly modulatable laser, the modulated laser light is reamplified in a solid state amplifier or fiber amplifier.
Also, with reamplification of the laser light, it is advantageous that the selection of the two wavelengths xcex1 and xcex2 or of the two polarizations P1 and P2 is carried out after the output of the amplifier stage.
In a further development of the invention, the resonator mirror and the out-coupling mirror are constructed as wavelength-selective or polarization-selective controllable reflectors and the active medium generates radiation of two wavelengths xcex1 and xcex2 or the active medium generates radiation of one wavelength with two polarization directions P1 and P2. This arrangement is a directly modulatable laser in which the two-wavelength method or the two-polarization method is combined with the two-direction method.
The controllable reflector is constructed as a volume-optical or integrated-optical component.
The invention accordingly relates to a directly modulatable laser and a directly modulatable reamplified laser in which at least one of the two mirrors forming the laser cavity is constructed as a controllable reflector. The controllable reflector serves as a wavelength changeover switch for the two-wavelength method, as a polarization changeover switch for the two-polarization method or as a light modulator for the two-direction method. In the two-wavelength method and two-polarization method, preferably only the resonator mirror is constructed as a controllable reflector. However, both mirrors of the laser cavity can also be constructed so as to be controllable.
In the two-direction method, the resonator mirror and the out-coupling mirror must be constructed so as to be controllable. When these two mirrors are constructed in such a way that they are wavelength-switchable or polarization-switchable, a combined effect of the methods is achieved in that the modulation depth is improved.
When using miniaturized or integrated-optical controllable reflectors, extremely high modulation frequencies of up to about 40 GHz and modulation depths of up to 40 dB can be achieved.
Both emission wavelengths or emission polarizations or emission directions are accordingly not supplied from external radiation sources, but rather are taken from the active ion-doped laser medium itself. In this regard, it must be ensured that the electron transition from the upper laser level to the lower laser level(s) is constant over time. This is equivalent to the demand that the inversion density is also constant over time.
The control of the electron transition is carried out in the two-wavelength method by controlling the individual electron transitions on the two selected emission wavelengths, in the two-polarization method by controlling the two polarizations, and in the two-direction method by controlling the light components of the two emission directions.
The amounts of the two emissions are given by the construction and control of the controllable reflectors and the emission spectrum of the active medium.
As a rule, one of the two emission wavelengths or emission polarizations or emission directions is used and the other is absorbed in a radiation trap. However, both emissions of the directly modulatable laser can also be utilized.
Further, by means of subsequent arrangement of an amplifier which, if required, is separated from the laser, e.g., by means of a Faraday isolator, it is possible to modulate both emission wavelengths or emission polarizations at low optical power (milliwatt range), to amplify them in the subsequent amplifier, and then to separate the utilized component from the component that is not utilized. A possible power limiting of the reflector components is avoided in this way and the high-power range of several watts can be included. Due to the fact that the amplifier is constantly kept in saturation by the two-wavelength operation or two-polarization operation, a particularly high contrast ratio can be achieved since the amplified spontaneous emission is also suppressed in the amplifier. The separation of the two emission wavelengths or emission polarizations is carried out at the amplifier output.
By constructing the laser as a fiber laser, the amplifier as a light-conducting fiber amplifier and the controllable reflectors as integrated-optical or miniaturized-optical components, a high degree of integration can be achieved and the component group or subassembly is essentially not prone to interference and requires no adjustment.
The controllable reflector is based, for example, on one of the following principles which are controllable: interferometer, absorber, light-path switching based on electro-optical, acousto-optical, thermo-optical, photothermal modulation or injection or depletion of charge carriers in waveguides. Further, liquid-crystal modulators are used. Another possibility is the generation of a periodic change in the geometric shape of the waveguide, e.g., as controllable amplitude gratings.
In particular, it is extremely advantageous that the controllable reflector(s) is (are) optically coupled directly with one end of a fiber laser or fiber amplifier, or two of these controllable reflectors are optically coupled respectively with one of the two ends of a fiber laser.
The invention will be explained more fully in the following with reference to the Figures.